Reference Frames

This is an inventory of the most important reference frames used in Cesium for Unreal.

Ellipsoid-centered

This is the "native" reference frame of Cesium.

Georeferenced

A reference frame defined by the ACesiumGeoreference actor.

Cesium Tileset

The reference frame of a Cesium 3D Tiles tileset, as defined by the tileset.json. Usually, tilesets are georeferenced and this reference frame is identical to the Ellipsoid-centered frame described above, but this is not strictly required by 3D Tiles. A non-georeferenced model of a building, for example, may have an origin at the center of the building and axes aligned with the principal sides of the building.

Unreal Tileset

The same as the Cesium Tileset, but expressed in Unreal Engine terms: the coordinates are in centimeters (multiplied by 100.0) and left-handed (have an inverted Y component) relative to Cesium Tileset coordinates.

Please note that the transformation from Unreal Tileset coordinates to Unreal Absolute/Relative World (below) is affected by the ACesium3DTileset Actor's Location and Orientation properties. However, these should almost always be set to identity.

Unreal Absolute World

Vectors and matrices in Unreal Engine are expressed using single-precision floating-point numbers. In order to maintain precision, these coordinate values must remain relatively small. To support this, the Unreal absolute world origin can be moved by setting the OriginLocation property of UWorld. Coordinates that are said to be in the "Unreal Absolute World" reference frame are expressed relative to the absolute origin (0,0,0) and are not affected by the value of the OriginLocation property.

Unreal Relative World

This reference frame has the same orientation as "Unreal Absolute World", but is offset from it by the UWorld's OriginLocation. Coordinates are expressed relative to the floating origin.